Inspection apparatus



S'ept. 19, 1961 H. G. NEIL INSPECTION APPARATUS Filed Jan. l1, 1956 4 Sheets-Sheet 1 PH OTOE LCTFUC DETECTO R.

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INSPECTION APPARATUS Filed Jan. ll, 1956 4 Sheets-Sheet 2 84 ,E56 fFLuOnEscEN-r -ruBE COUNTER. (HOLE) PULSE HEIGHT DISCRIMI NATOR POSITIVE PHOTO CELL i/AMPLIFIER.

PULSE HEIGHT mscmMmArOR NEgA'rlvE AMPLIFIER ELECTRIC CLUTCH PULSE HEIGHT PULSE HEIGHT DISCRIMINATOR PHOTO POSITIVE CELL AMPLIFIER PULSE HEIGHT DISCRIAI q TOR NE A VE CO NTEFL (gms, lNvENTOR DISCRIMINATOR PHOTO POSITIVE CELL AMPLIFIER LIGHT METER HUGH G. NEIL BY M ATTORNEYS Sept. 19, 1961 H. G. NE1L 3,001,080

INSPECTION APPARATUS Filed Jan. 1l, 1956 4 Sheets-Sheet 3 1N VENTOR HUGH G. NEIL ATTORNEY Sept. 19, 1961 H. G. NEIL INSPECTION APPARATUS Filed Jan. 1l, 1956 4 Sheets-Sheet 4 IN VENTOR HUGH G. NE I L ATTORNEY United States Patent 3,001,080 .INSPECTION APPARATUS Hugh G. Neil, Knoxville, Teun., asslgnor to Special Instruments Laboratory, Incorporated, Knoxville, Tenn., a corporation of Tennessee Filed Jan. 11, 1956, Ser. No. 558,529 16 Claims. (Cl. Z50-219) This invention relates to inspection apparatus, and more particularly, to apparatus 'for inspecting web materials such as woven clot-h.

Heretofore, the inspection of cloth has been carried out visually by employees especially trained for this job. The cloth was examined for flaws las it was fed past a viewing station on an inspection frame. When a iiaw was detected, the operator would stop the feeding of the cloth so that the iiaw `might be repaired if possible. Then the feeding of the cloth would be resumed.

This process is subject to several disadvantages. It is tedious from the point of vieW of the operators of the inspection frames, and it is expensive and unreliable from the `point of view `of the plant management.

It is an object of the ,present invention to provide apparatus for automatically inspecting web material, such as cloth, so that visual examination can be dispensed with and so that uniformity of inspection standards can be assured.

It is a further object of this invention to provide photoelectr-ic web inspection apparatus for detecting `flaws which are `very `small as `compared tothe width of the web being inspected. Morepparticularly, it is desired in connection with the inspection of print .cloth that the detector react to flaws such as slubs and holes.

Another object of `the `present invention -residesin the provision of `web inspection apparatus Vhaving a aW detector and having suitable means for controlling the feeding of the web rin response to .signals .from `the detector.

Still another object of the ,invention` is the provision of inspection apparatuscapable of-detecting notonly aws in the material being inspected, but also the overall thickness of the material. Inthe case of fabrics, the appara tus gives, in addition to indications of the s lubs andholes appearing in the fabric, an indication of weight of a cross section of the fabric.

It is -a morespecilic objectof this invention to provide photoelectric apparatus having a durable light source capable of giving uniform light over a substantial -width of material to be inspected.

The `foregoing objects are attained, according `to one embodiment of Vthe present invention, by an apparatus having web `feeding means, web take-up means, and photoelectric detection means therebetween. An electric brake is associated with the feed means and `is counected to the detector so that it is `operated to stop `the feeding of the `fabric upon the detection of a aw. The take-up means normally is driven through an electric clutch, but this clutch is operative upon `the detection of a aw to disconnect the take-up means from its drive. Flaw counters and a web thickness meter 'also may be included in the apparatus.

The photoelectric detector receives light from a light source in the form of an arc discharge fluorescent tube. This tube is energized by a radio frequency alternating current so as to maintain a steady light along the entire length of the tube. An inductance is connected to the tube to uncouple the severe tube fluctuations due to arcing from the radio frequency source.

The `detector itself is sensitive to flaws of small mag-` nitude. This `is accomplished by the-use of ,a number of detector heads arranged yacross the width of Vthe `detector, with each serving to scan a small area. The out puts from the several detector heads `are not combined immediately. -First a determination is made by suitable electric circuit means lfor each detector to ascertain whether or not a flaw appears in the section of fabric scanned by that detector. IIn this Way the signal-to-noise ratio `is high `in spite of 'the relatively small aws being detected. p

The electric -circuits employed include several features of particular significance. Among these is the fact that the photoelectric cells are so connected to their `amplifiers that tthe incidence of a very large amount of light upon a cell will serve `to remove that cell from the aw detecting circuits. 'In Way the fact that the cloth does not cover the endmost cells of the group in certain instances is not a serious defect.

Another feature of the electric circuit used in connectionwith the detector of the present invention is that the initial amplifier stages for the signals emanating from the `photoelectric` cells are held within their normal operating ranges by lthe novel expedient of connecting all of their cathodes to ground through a common cathode resistance.

A better understanding of the invention and `its many advantages will ibe gained from a consideration of the following detailed description of certain embodiments thereof illustrated in the accompanying drawings, in which:

FIG. l is a diagrammatic perspective view of Ian inspection machine according tothe present invention;

FIG. 2 is a y,View `taken `alo-ng the line 2 2 in FIG. 1. showing the upper surface of one form of photoelectric detector;

FIG. 3 is a cross sectional view taken along the line 3 3 in FIG. 2 and showing the relative positions ofthe light source, the web material being inspected, and `the photoelectric detector;

:FIG. 4 .is a cross sectional view of the detector taken along the line 4-4 in FIG. 2;

=FIG. 5 vis Ia cross sectional view ofthe detector taken along the line S-S n FIG. 2;

(FIGS. 3, 4, and 5 havelbeen rotated counterclockwise approximately FIG. 6 is a diagram of an electric circuit for energizing `the fluorescent tube which serves as a light source lfor the inspection apparatus of the present invention;

FG. '7 is Ia Ablock diagram indicating the operations which take Aplace in response `to signals from `the detector;

FIG. 8 is a diagram of the electric circuits for utilizing the signals developed in the detector heads;

FIG. V9 is a view similar to lFIG. 2, but showing auother embodiment `of the detector;

FIG. l0 is a cross sectional View taken along the line lll- 10 of FIG. 9; and

FIG. 1l is a cross sectional view taken `along the line 1i1-11 in BIG. l() with the electrical components inend elevation.

The inspection machine, diagrammatically illustrated in FIG. l, includes a roll Ztof the material to be inspected. This material may be Woven fabric, knit goods, webs of paper, or the like. The supply `roll 2 is mounted in any suitable lfashion upon a shaft 4 attached to an electric brake 6. The `cloth 8 or other material from the supply roll 2 passes over idler rolls 10 and 12 and then is `wound upona receiving roll 14. In the case of woven fabrics, it is preferred that the Warp threads extend lengthwise of the advancing fabrics. The roll 14 is `mounted upon `a shaft 16 driven by an electric motor 18 through a suitable electric clutch 20. No attempt has been made in FIG. 1 to illustratethe framework ofthe machine because the particular `structure employed is of `no-importancerto the present invention. Anyrof the well known inspection frames of the prior art may be used if desired.

As the cloth 8 passes between the idler rolls 10 and 12, it is examined for irregularities by the photoelectric inspection mechanism of the present invention. The numeral 22 is applied in FIG. l to indicate generally a light source disposed above the traveling cloth. Light from this source passes through the cloth to a detector designated generally by the numeral 24.

The structure of the detector 24 is illustrated in greater detail in FIGS. 3, 4 and 5. The upper surface of the detector 24 is covered by a metal plate 26 provided with two rows of slots 28 and 30. 'Each of these slots may be on the order vof two inches in length, and each is preferably of a width corresponding to the maximum width of the defectsto be detected by the apparatus. In the case of cloth inspection,A the width of each of the slots 28 and 30 should correspond to the thickness to be expected of a slub in the fabric. As indicated in FIG. 2, the slots 28 and 30 are staggered and overlapped so that together they permit inspection along the entire width of the fabric.

Beneath the plate 26, the detector 24 is in the form of a sandwich comprising outer layers 32 and 34 of aluminum or other suitable opaque material, intermediate layers of a transparent plastic 36 and 38, and a central layer 40 of an opaque material. The bottom of the detector 24 is closed by an opaque plate 42, and the ends, by opaque plates such as 43 (FIG. 5

Each of the blocks of plastic 36 and 38 is shapedin the manner indicated in FIG. to provide lens portions 44 beneath each ofthe slots in the upper plate 26 and to provide spacers 46 intermediate the lenses 44. The straight sides of the spacers 46 serve as shoulders for strips 4S of aluminum or some other opaque material. These strips 48 have a lateral extent corresponding to the thickness of the plastic with which they are associated. For example, the strips 48 associated with the plastic block 38 extend between the side plate 34 and the centrally located divider plate 40.

The arrangement is such that the space beneath each one of the slots 28 and 30 forms a light chamber into which light may be admitted only through the slot at the top. Near the bottom of each of the light chambers thus formed, there is located a photoelectric cell. VThese cells conveniently are provided in two sizes so that they may all extend to one of the faces of the assembly 24.

The cells 50 for the light chambers beneath the slots 30 are short, and they extend outwardly from the light chambers through the side wall 32. It will be understood in this connection that no light is allowed to enter the light sensitive portions of the cell 50 from the exterior of the assembly, however.

The photoelectric cells 52 for the light chambers located beneath the'slots 28 are elongated somewhat, as indicated in FIG. 3. These cells pass through the centrally located wall 40 and the side wall 32. Again, it should be pointed out that only light from the chamber beneath a slot 28 may affect the cell 52.

The light source 22, shown in FIG. 3, includes -a reflector 54 and a light emitting element 56. The requirements imposed upon this light source `are unusual and they will be considered in detail.

In View of the fact that frequencies produced by light shining through a slub are on the order of 200 to 3,000 cycles per second, depending upon the size of the slub and speed of the cloth, and in view of the fact that it is desirable to measure the yard by yard uniformity of the cloth, the light falling on the cloth must be of constant brilliance and without ripple to a frequency of 0 to 20,000 cycles. Such light may be derived from well ltered incandescent lamps, but the obtaining of a uniform intensity across the width of the cloth by means of a lens system is both cumbersome and expensive.

Moreoven the use of a iluorescent ylamp as the light emitter 56 is subject to serious disadvantages if such a lamp is operated on direct current. After direct current operation of the lamp for several days, two diiculties usually arise. First, one end of the tube becomes dark, and a flicker on the order of 500 to 2,000 cycles per second appears in this region. This flicker is of considerable magnitude so that it masks the signals produced by the defects in the cloth. Second, continuous oscillations of smaller magnitude and a frequency on the order of 1,000 cycles per second develop in the tube. Y

According to the present invention, these diiiculties are overcome by the use of a iiuorescent tube energized by a radio frequency source. Since the upper limit of the signal frequencies to be expected is on the order of 5,000 cycles per second, an are discharge lamp driven by a radio frequency source in the range of 50 to 100 kilocycles per second is entirely satisfactory.

It will be evident that this feature of the invention is not necessarily limited to web inspectors. It can be used in a variety of applications, and is particularly advantageous in those situations where relatively low frequencies normally are imposed upon the outputs of light sensitive devices.

The oscillator shown in FIG. 6 is conventionalin the art, and its operation need not be described in detail. It comprises a pair of electron tubes 60 and 62 connected together as shown in a circuit containing resistances 64, 66, and 72, and capacitances 68, 70, 74, 76, and 78, of suitable values. The power supply should furnish direct current at about 400 volts.

The output of the oscillator is through a transformer 80, the secondary 82 of which is connected across the uorescent tube 56 to supply suiiicient potentials to the tube 56 to cause it to operate as an arc discharge device. In order to prevent the firing peak from the fluorescent tube S6 from being fed back into the oscillator through the transformer S0, an inductance 84 of, for example, 2.5 millihenrys is utilized inthe uorescent tube circuit. This inductance 84 is important to the successful operation of the circuit because the extreme fluctuations which occur incident to the arcing of the tube 56 must not be allowed to interfere with the oscillator. In effect, the inductance 84 serves to uncouple the oscillator and the uo'rescent tube 56 as far as the extreme uctuations are concerned.

The general nature of the inspection operation carried out by the apparatus described above will be evident. Light from the source 22 passes through the material being .inspected and into the slots 28 4and 30. The amount of light entering the slots 28 and 30 depend-s upon the shadow characteristics of the material being inspected. As the material becomes more dense, less light will be transmitted to the slots, and vice versa.

These variations in light intensity are evaluated and utilized in the apparatus of this invention in the manner indicated by the block diagram in FIG. 7. There are only three photocells in this diagram, but this represents merely `an attempt to simplify the disclosure. In an actual embodiment of a cloth inspector, there might be thirty or more of these photocells connected in parallel.

For one type of evaluation, the signals from the several photocells may be combined almost immediately, if desired. As shown, all of the cells `are connected to a light meter which is caused to indicate the average of the light intensities at the several cells. This average is a measure of the overall density, or weight per yard, of the material being inspected.

` For the detection of aws, however, it is not desirable to combine the several photocell signals until after they have been evalua-ted independently. This technique assures a high signal-to-noise ratio, which Simplies the circuits and increases the reliability of the equipment. Consequently, the signal from each cell is amplied and passed to pulse discrrninators which serve to distinguish between positive and negative variations, and which serve also to block the passage of `all those signal components having magnitudes below the levels corresponding to the types of flaws to be expected. Thus, the outputs `from the several pulse discriminators represent definitellaws in the goods being inspected.

A flaw pulse delivered by any of the pulse discriminators is fed lto one of a pair of amplifiers, and then is delivered to a counter mechanism, the electric clutch 20, and the electric `brake 6. The counters provide records which are valuable in connection with the quality control procedures of the plant, and the clutch and brake 6 4function to stop the feeding of the cloth upon the discovery of a flaw so that the flaw may be repaired if possible.

In the circuit diagram of HG. 8 `only two of the many photocells Sti and 52 of the detector -of 4the present invention have been shown. Each ofthe photocells has associated with it certain circuit components which are connected in parallel with the corresponding components associated with the other photocells. This arrangement is illustrated in FIG. 8 where the components associated with the cell 52 bear reference characters `which correspond, except for addition of the letter a, to those applied to the components associated with the `cell 5ft. The dotted lines 86 indicate the relative position of :the omitted branches of the circuit.

'I'he cathodes of all of the photocells (50 land 52 in the detector 24 are supplied with a negative potential, of about -l48 volts by a line 88, variablelresistances 90 and 92, a filter system m-ade up of an inductance 94 and capacitances 96 and 98, a rectifier 100, and a transformer 102.

The anode of the photocell 50 is connected to ground through a resistance 104, which should be calibrated for the sensitivity of the cell 50, and through a capacitance 106. The capacitance 106 serves to short out the radio frequency portion of the signal from the photocell, which portion may be attributed to the above-described radio frequency excitation of the light source 56.

As thus modified, the signal from photocell 50 is led to the grid of a first amplifier stage 108. The cathode of this stage 188 is connected through cathode resistances 110 and 112 to the negative voltage supply described above. Positive potentials are supplied to the anode of amplifier stage 108 through resistances 1'14 and 115, a variable resistance 116, a filter system made up of capacitances 118 and 12dY and inductance 122, a rectifier 124, and a `transformer 126.

Preferably, voltage regulators 128 and 130, with suitable capacitances 132 and 134, are connected as shown to the positive and negative potential supplies to minimize fluctuations.

One significant feature of the portion of the circuit described above is the relationship between the photocell 5@ and theampliiier Stage 108. The cell 50 is of the type in which the conductivity increases with an increase in the amount of light falling on the cell. Consequently, the effect produced When an excessive amount of light falls on the cell 5d is a sharp 4lowering of the potential at the grid of the amplifier stage 108. As the grid becomes more negative with respect to the cathode of the stage 168, this stage is effectively blocked or shut off.

This feature is significant primarily in connection with the scanning of the edge portions of the cloth. It sometimes happens that `the cloth `does not cover completely the endmost of the slits 28 and 30 in the detector 2'4, and the photocells under these slits give signals corresponding to those which would result from very large holes in the fabrics. These unwanted signals are kept out of the flawregistering portion of the circuit Iby the blocking of the first amplifier stage 108.

It should be noted .that this effect could not be obtained if the photo-cell 50 were used in the conventional Way, with the output signal being taken from the cathode of the cell. As the grid of the amplifier became excessively positive under the influence of a greatly increased amount of light, the amplifier would respond with a signal of large magnitude which ultimately would cause the stopping of the fabric feeding means and which would be recorded as a hole by the Iappropriate `counter mechanism.

Another significant feature of the circuit which should be particularly pointed out isA that `resistances i1410 and 112 for the cathode of the first'arnplifiE-stage 108 are common to the cathodes of lall of the first amplier stages 108, 168:1, etc. `in the system. Connecting all of these cathodes through the s ameresistance path has the advantage that the potentials of all of these remain at about the same level, even though one of them may momentarily pass more current The result ,is that an automatic levelling effect is produced, and all of the initial amplifier stages vare held Within their optimum operating ranges.

With this condition `being maintained, the amplification given a slub or holesignal is exaggerated, and at the same time, the effect produced by a condition which extends across the entire width of `the fabric is mini- Inized. Thus, this portion of the circuit is sensitive lto flaws rather than to overall variations in fabric Weight,

such as might be caused `by ydifferent weights of the `weft t weights.

After being amplified by the amplifier stage 108, the signals originating in the photocell 50 pass through a resistance 136 to a second amplifier stage 138, which is preferably a cathode follower. As illustrated, the stages 108 and 138 occupy the two halves of a twin triode `140. This is a convenient arrangement, but obviously it is not essential, The cathode of the stage 138 is connected to ground through a cathode resistance i142, and the anode is supplied with a positive potential through resistances 144 and 146.

The output from the second amplifier stage 138 is coupled to a pulse discriminator 148 by a capacitance 15G and a resistance 1152. The pulse discriminator 148, shown in FIG. 8, is a twin diode tube, but it could be formed from a pair of separated diodes. The signals from the amplifier stage 138 are fed to the cathode of a first diode 154 and to the anode of the second diode 156 of the pulse discriminator '148.

It is a function of the discriminator to evaluate the signals received by refusing to transmit any pulses unless the incoming signals are of `magnitudes indicative of flaws in the material being inspected. The nature of each flaw also is ascertained Iby the pulse discriminator 148 in that a hole in the fabric Will cause the diode 154 to fire and a slub will cause the vdiode `156 to fire.

These results are obtained by maintaining the anode of the diode 154 negative, and the cathode of the diode 156 positive. The anode ofthe diode `154 is connected through a resistance 158 to the movable contact of a potentiometer 160, one side of which is connected to ground and the other side of which is connected to the negative potential source through a resistance I162. The cathode of the diodev156 is connected through a resistance 164 to the movable contact of a potentiometer 166, one side of which is connected to ground :and the other side of which is connected through a resistance 168 to the positive potential source.

A pulse from the diode 154 passes to an amplifier stage 170 through a resistance 172, and a pulse from the diode 156passes to an amplifier stage 174 through a resistance 176. The stages 170 `and 174 may bein the 'form of a twin triode tube 178, as illustrated. The cathodes of stages 170 and V174 are connected to ground through capacitances and `182, respectively, and ,to the negative potential source `through resistances 184 `and '186, respectively. The anodes of kthese stages are connected tofthe positive potential` supply through resistances"188 and I190, respectively.

From `the amplifier stage 170, ahole signalpasses to 7 a control grid of a thyratron 192 through a coupling composed of a capacitance 194 and resistances l196 and 198. A slub. signal, on the other hand, passes through a phase inverter stage 174 to a control grid of a thyratron 200 through an intermediate tube 202. The coupling between the tube 202 and the amplier stage 174 consists of a capacitance 204 and resistances 206 and 208. The cathode of the tube 202 is connected to the source of negative potential through a resistanceV 210, and the anode is connected to the source of positive potential through a resistance 212.V The coupling between tube 202 and thyratron 200 consists of a capacitance 214 and resistances *216 and 218.

Each of the thyratrons l'192 and 200 includes a grid in addition to the gr-id to which the incoming signals are fed, and these Vadditional grids are connected through resistancesY 220 and 222 to the movable contact of a potentiometer 224. One side of the potentiometer 224 is connected to ground, and the other side is connected through a resistance 226 to the source of negative potential.

The anode circuit of the thyratron 192 is connected to the power supply through the coils of relays 228 and 230, and an RC coupling composed of a resistance 234 and a capacitance 236. The anode circuit for the thyratron 200 is similar. It includes the coils of two relays 238 and 240.

The relays 228 and 238 control the circuits of the hole and slub counters, respectively, In FIG. 8 the numeral 244 has been applied to an element representing a coil in the hole counter mechanism 245, While the numeral 246 has been applied to a corresponding coil in the slub counter 247. It will be understood that the counter circuits illustrated are purely diagrammatic. These may take any convenient form.

The relays 230 and 240 each have a pair of contacts. One contact of each pair is in a holding circuit, which serves to bypass the thyratron after the thyratron has once tired. These holding circuits for the coils of relays 230 and 240 also include resistances 248 and 250, respectively. The remaining contacts are connected in parallel with each other, as shown, in a circuit which includes a coil 252 associated with the electric brake 6 and a coil 256 associated with the electric clutch 20. Upon the closing o-f one of the relays 230 or 240, the brake 6 tends to stop the feeding of the fabric 8, and the clutch i20 disconnects the motor 18 from the take-up roll 14. After a flaw has been repaired, the feeding of the fabric may be resumed by manually opening the holding circuit of the appropriate one of the relays 230 and 240.

As mentioned earlier in this description, it is desirable to include in a cloth inspector, means for indicating the average thickness of the cloth. This may be accomplished by connecting all of the lphotoelectric cells 50 and 52 in parallel, as described, and by connecting the whole group to a suitable meter circuit.

A signal representing the overall light conductivity of the fabric being inspected yappears across the variable resistor 90. This signal is ampliiied and evaluated by a meter circuit comprising two tubes 260 and 262 and a meter 264. If desired the meter 264 may include means, such as a stylus, and a continuously advancing web of paper, `for recording continuously the fabric thickness measurements obtained.

In the illustrated meter circuit, the cathodes of the tube 260 are connected to each other and to the negative potential source through a resistance 266. The two grids of the tube 260 are connected to opposite sides of the variable resistance 90, and the anodes are connected to the positive potential'source through loading resistances 268, 269, and a tapped resistance 270.

The -auodes of the tube 260 are tied together, and the cathodes are connected to the negative potential source through resistances 272 and 274. The meter 264 is connected between the' cathodes and has a protecting resistance 276 in series therewith.

The illustrated circuit is 278, 2,80, and 282 which serve, respectively, to iilter AC from the two positive potential lines and the negative potential line. Y

FIGURES 9, and ll illustrate a detector 284 which may be employed instead of the detector 24 described above in connection with FIGURES 3, 4 and `5. V

The detector 284 includes a pair of transparent plastic layers 286 and 288 which are similar in shape to the plastic layers 36 and 38 of FIGS. 3 to 5. Each includes lens portions 290 with intervening spacer portions 292, and the lens portions 290 of one of the transparent layers 286 and 288 is disposed opposite the spacer portions 292 of the other of the layers. The lower surfaces of each of the spacer portions 292 is provided with an opaque coating 293, such as black paint for example.

The transparent layers 286 and 288 are held in position between a central metal plate 294 and a pair of metal reinforcing members or side Vplates 296 and 298 to provide a sandwich structure. In assembling the sandwich, bolts 300 tirst are inserted into countersunlr holes in the central plate 294. Then, the central plate 294-, the plastic layer 286, and the side plate 296 are connected together by screws 302, which iit into countersunk holes in the central plate y294 and which are threaded into Vthe side plate 296. .NeXt, the plastic layer 288 andthe side plate 298 are ttedonto the protruding ends of the bolts 300 and secured together by nuts 304 which threadedly engage the end portions of the bolts. The bolts 300 and the screws 302 pass through spacer portions 292 of the plastic layers 286 and 288 so that they do not interfere with the collection of light by the lens portions 290.

VBelow the center of each of the lens portions 290 of the plastic layers 286 and 288, the central plate 294 is provided with a hole 306 for the reception of a photocell 308. These photocells correspond in function to the photocells 50 and 52 of FIGS. 3 and 4. However, they are all the same size, and they are all mounted in the same way.

The photocells 308 -are shielded from extraneous light by covers 310 and 312 which may be attached to the side plates 296 and 298 by screws 314. The lower edges of the covers k310 and 312 may simply abut against the central plate 294, as illustrated, or, if desired, they may be secured thereto by any suitable means. The cover 312 is formed with sockets 316 for the ends of the several photocells 308.

It should be noted that, in this embodiment, there are no elements corresponding to the top plate 26 and the strips 48 of PGS. 2 to 5 and that each ofthe photocells 308 is exposed on both sides of the central plate 294. In these respects the physical separation of the several light chambers is less complete. It has been found in practice, however, that the opaque coatings `293 and the focusing effected by the lens portions 290 result in adequate separation of the light rays so that the only significant amounts of light reaching a given photocell 308 are those which pass through the lens portion 290 immediately above it.

Below each of the photocell openings 306, the central plate 294 is provided with a pair of sockets 318. Each of these soclcets receives an electron tube 320, which corresponds to one of the tubes or 148 indicated in FIG. 8. With this arrangement, it is possible to locate the electrical components directly associated with the photocells 308 in the detector 284.

A sheet metal housing 322 is attached to the cover 312 by screws 324 or other suitable means. This housing may tit against the lower end of the central plate 294 in the manner illustrated in FIG. lO, or it may be secured to the central plate in any convenient way. The space defined by the housing 322 accommodates the wiring and the small circuit components, such as resistors and capacitors, associated with the photocells 308 and the tubes Although certain embodiments of this invention have completed by capacitances amigos@ been described in detail, various alterations and modifications will be apparent to persons skilled in the art. Moreover, other combinations and fields of utility for certain of the features of the invention will be obvious. Consequently, it is intended that the foregoing description be considered as exemplary only, and that the scope of the invention be ascertained from the following claims.

I claim:

Vl. Apparatus `for determining the existence of slubs or holes in cloth comprising means -for advancing the cloth along a path, a plurality of separate sensing units arranged across the Width of the cloth in overlapping relation so as to cover the entire width ofthe cloth, each of said units being operable to sense the condition of the cloth over an area the width of which is small enough that a signal of substantial magnitude is produced by the passage of a slub or hole through said area, and separate discriminator means operatively connected to each of said sensing units 'for distinguishing between slub or hole signals and signals of lesser magnitudes.

2. Apparatus for determining the existence of slubs and holes in cloth comprising means for advancing the cloth along a path, a plurality of separate electric sensing units arranged across substantially the entire width of the cloth and responding to density variations in the cloth by changes in their electrical condition, each of said units being responsive to the density of a segment only of the width of the cloth, a plurality of rst unidirectional current flow devices having a positive terminal and a. negative terminal, a plurality of second unidirectional current flow devices having ra positive terminal and a negative terminal, circuit means operatively connecting each of said sensing units to the negative terminal of one of said first and to the positive terminal of one of said second unidirectional current flow `devices so that the changes in the electrical condition of a sensing unit are reflected in changes in the potentials of such terminals, means for imparting a negative bias to the positive terminal of said iirst ow device, means for imparting a positive bias -to the negative terminal of said second flow device, iirst counter means operatively connected to all of said rst unidirectional iiow devices for recording the number of occasions on which currentliows in such rst devices, and second counter means operatively connected to all of said second unidirectional ilowdevices for recording the number of occasions on which current flows in such second devices.

3. Apparatus for examining woven fabrics comprising means for advancing a fabric along a path with the Warp threads of said fabric extending generally parallel to said path, light-sensitive means on one side of said path, an electric `light source on the other side of said path opposite said light-sensitive means, and circuit means for supplying radio frequency energizing current to -said light source of such a high frequency related to the frequencies of the light variations resulting from the passage of light through the spaces between the threads of the fabric that said light source will not ilicker at a frequency of an order close to the frequencies of the light variations resulting `from the passage of light through the spaces between the threads of the woven fabrics.

4. Apparatus for determining the existence of slubs and holes in cloth comprising means for advancing the cloth along a path, light-sensitive means on one side of said path, a fluorescent tube extending across the entire width of said cloth on the other side of said path at a position opposite said light-sensitive means, circuit means for supplying a radio frequency energizing current to said liuorescent tube, and capacitance means operatively associated with said light-sensitive means for shorting out that portion of the electric signal from said lightsensitive means attributable to the radio frequency energization of said tube.

5. Apparatus for inspecting material for localized irregularities comprising means `for advancing the Ina- `ves terial along a path, a plurality of separate electric sensJ ing units arranged across the width of the material and responding -to density variations in the material by changes in their electrical condition, a plurality of iirst amplier stages each comprising a negative element, a positive element, and a control element, means connecting each of said sensing units to the control element of one of said `amplifier stages, and a common resistance path for the negative elements of all of said rst amplier stages, whereby the ampliiication given by an ampliiier stage to a signal resulting from an irregularity sensed by less than all of said sensing units is greater than that given to a signal resulting from a condition sensed by all of said sensing units.

6. Apparatus for inspecting material comprising a plurality of electric sensing units arranged across the width of the material and responding to density variations in the material by changes in their electrical conditions, a

'plurality of irstunidirectional current flow devices for amplifying the variations in the electrical conditions of said Sensing units, each of said devices comprising a negative terminal, a positive terminal, and a control terminal, and circuit means operatively connected to one of said sensing units and the control terminal of a unidirectional current dow device in such a way that the potential of said control terminal becomes negative and blocks the passage of current through such device in response to` an electrical condition in the sensing unit corresponding to an extremely low density.

7. Apparatus for inspecting cloth comprising means for advancing the cloth along a path, a light source on one side of said path, a plurality of photoelectric sensing units arranged across the width of the cloth on the other side of said path opposite `said light source, circuit means for applying a negative potential to the cathode of each of said photoelectric sensing units and for connecting the anode of each unit to ground, an electronic ampliiier for each of said sensing units, each of said amplifiers including a control element, and means for connecting the anode of each sensing unit to the control element of the associated amplifier in such a Way that an extraordinarily large drop in the potential of the anode will lower the potential of the control element suiiciently to block substantially the passage of current through the amplier.

8. Apparatus for inspecting cloth comprising means for advancing the cloth along a path, a light source on one side of said path, a plurality of photoelectric sensing units arranged across the width of the cloth on the other side of said path opposite said light source, circuit means for applying a negative potential to the cathode of each of said photoelectric sensing units and for connecting the anode of each unit to ground, an electronic `ampliiier :for each of said sensing units, each of said ampliers including a control element and a cathode, means for connecting the anode of each sensing unit to the control elementof the associated amplifier in such Va way that an extraordinarily large drop in the potential of the anode will lower the potential of the control ele-ment sufficiently to block substantially the passage of current through the amplier, and a com-mon resistance path for lthe cathodes of all of said electronic amplifiers.

9. In apparatus for inspecting material, the .improvement which comprises a photoelectn'c detector including a central member of opaque material, first and second transparent members on opposite sides of said centra-l member, each of said transparent members including a series of spaced lens portions and intervening opaque spacer portions with the lens poitions of one of said transparent members being disposed opposite the spacer portions of the other of said transparent members, a light- `sensitive element :disposed beneath each of said lens portions, and cover means for preventing the passage of Ilight to said light-sensitive elements except through said lens portions.

` Cloth" inspection apparatus for determining the existence of flaws in the nature of localized density irregularities of predetermined minimum magnitude comprising means for advancing the cloth along a path, a plurality of separate electrical sensing units arranged across substantially the entire Width of the cloth passing along said path and responding to density variations in the `cloth by changes in their electrical condition, each of said units being operable to sense the condition of a segment only of the width of the cloth, a plurality of electrical discriminator means each operable to distinguish between` electrical signals having a magnitude above a predetermined value and other electric signals, circuit means connecting each of said sensing units to a separate one of said discriminator means for transmitting to each of the discriminator means signals from one only of said sensing units so that the magnitude of each such signal is evaluated independently, and means connected to all of said discriminator means for stopping the advance of the cloth in response to actuation of any one of said discriminator means.

11. Cloth inspection apparatus for determining the existence of slubs and holes in cloth comprising means for advancing the cloth along a path; a plurality of lightsensitive units arranged across substantially the entire Width of the cloth passing along said path and responding to density variations in the cloth by changes in their electrical conductivity, each of said units being operable to sense the condition of a segment only of the Width of the cloth; a control circuit including means for stopping the advance of the cloth and a pair of control devices connected in parallel so that actuation of either of said devices will cause actuation of said means for stopping the advance of the cloth; a group of iirst electrical discriminator means connected to one of said control devices so that actuation of any one of said first discriminator means will cause actuation of such control device, each of said first electric discriminator means being operably connected to one only of said light-sensitive units and each being responsive only to changes in the conductivity of the light-sensitive unit connected thereto corresponding to the movement of a slub passing such unit; and a group of second electrical discriminator means connected to the other of said control devices so that actuation of any one of said second discriminator means will cause actuation of such control device, each of said second electrical discriminator means being operably connected to one only of said light-sensitive units and each being responsive only to changes in the conductivity of the light-sensitive unit connected thereto corresponding to the movement of a hole passing such unit.

l2. Cloth inspection apparatus comprising means for advancing cloth along a path, detector means operable as said length of cloth passes along said path to sense density variations in the cloth, first evaluator means operably connected to said detector means and operating to determine what density variations sensed by said detector means correspond to slubs or holes, second evaluator means operably connected to said detector means to measure the average thickness of said cloth over an area extending substantially from one side edge to the other side edge of said cloth, means connected Y to said second evaluator means for indicating the thickness of the cloth, and means connected to said rst evaluator means and operating in response to the detection of either a slub or a hole.

i3. Cloth inspection apparatus for determining the existence of slubs and holes in cloth comprising means for advancing the cloth along a path; a plurality of lightsensitive units on one side of said path arranged across substantially the entire Width of the cloth passing along said path and responding to density variations in the cloth by changes in their electrical conductivity, each of said units being operable to sense the condition of a segment onlyof the width of the cloth; a fluorescent tube extending across the entire Width of said cloth'A on the other side l2 Y Y of said path at-a position opposite said light-sensitive means; circuit means for supplying radio frequency energizing current to said fluorescent tubeg'a control circuit including means for stopping the advance ofthe cloth and a pair of control devices connected in parallel so that actuation of either of said devices will cause actuation of said means for stopping the advance .of the cloth; a group of rst electrical discriminator means connected to one of said control devices so that actuation of any one of said lirst discriminator means will cause actuation of such control device, each of said iirst electric discriminator means being operably connected to one only of said light-sensitive units and each being responsive only to changes in the conductivity of the light-sensitive unit connected thereto corresponding to the movement of a slub past such unit; and a group of second electrical discriminator means connected tothe other of said control devices so that actuation of any one of Vsaid second discriminator means will cause actuation of such control device, each of said second electrical discriminator means being operably connected to one only of said light-sensitive units and each being responsive only to changes in the conductivity of the light-sensitive unit connected thereto corresponding to the movement of a hole past such unit.

14. Cloth inspection apparatus for determining the ex istence of slubs and holes in cloth comprising means for advancing the cloth along a path; ya plurality of lightsensitive units arranged across substantially the entire width of the cloth passing along said path and responding to density variations in the cloth by changes in their electrical conductivity, each of said units being operable to sense the condition of a segment only ofthe width of the cloth; a group of rst electrical discriminator means each of which is operably connected to one only of said lightsensitive units and each being responsive only to changes in the conductivity of the light-sensitive unit connected thereto corresponding to the movement of a slub past such unit; and a group of second electrical discriminator means each of which is operably connected to one only of said light-sensitive units and each being responsive only to changes in the conductivity of the light-sensitive unit con.- nected thereto corresponding to the movement of a hole past such unit.

15. Clothrinspection apparatus for determining the existence of slubs `and holes in cloth comprising means for advancing the cloth along a path; a plurality of lightsensitive units arranged across substantially the entire width of the cloth passing along said path and responding to density variations in the cloth by changes in their electrical conductivity, each of said units being operable to sense the condition of a segment only of the Width ofthe cloth and to produce positive yand negative pulses in response to slubs yand holes; a group of rst electrical discriminator means each including a unidirectional current flow device operably connected to one only of said lightsensitive units and being responsive only to pulses from the light-sensitive unit connected thereto, and regulatable means for biasing each said flow device to prevent the passage of lsignals therethrough except when a positive pulse of at least a predetermined height is delivered by the light-sensitive unit connected thereto; and a group of second electrical discriminator means each including a unidirectional current How device o rably connected to one only of said light-sensitive units and being responsive only to negative pulses trom the light-sensitive unit connected thereto, and regulatable means for biasing each of last-mentioned ilovv devices to prevent the passage of signals therethrough except when a negative pulse of at least a predetermined height is delivered by the light-sensitive unit connected thereto.

16. Apparatus for examining textile fabrics comprising means-for advancing a fabric along a path, light-sensitive means on one side of said path, an arc discharge lamp on the other side of said path opposite said light-sensitive means, said lamp extending -across the entire width of the advancing fabric, circuit means for supplying energizing current to said lamp at a frequency above 20 kilocycles per second so that the lamp will not icker at a frequency of an order close to the frequencies ofthe light variations resulting from the passage of light through spaces between the threads of the advancing fabric, and amplifier means connected to said light-sensitive means and operating to amplify signals from said light-sensitive means having frequencies corresponding to those of the light variations resulting from the passage of light through the spaces between the threads of the advancing fabric.

2,075,200 abuka Mar. 3o, 1937 14 Huber Dec. 7, Furedy Nov. 3, Thomas Apr. 11, 'Hags Feb. 19, Hepp et al. Mar. 23, iB-assett Jan. 11, Marenholtz Aug. 1, Condlife et a1. Dec. 16, Shoemaker Apr. 14, Sweet June 9, Slayton Ian. 26, Daniels .Tune 5, Linderman Aug. 14, Mindheim Mar. 17,

FOREIGN PATENTS Great Britain Dec. 8, 

